Possible role of pH gradient and membrane ATPase in the loading of sucrose into the sieve tubes

Abstract

FOR plants to grow, sugars and other assimilates must constantly be transported from the leaves to the growing or storage regions. This essential transport function takes place in the highly specialised phloem tissue which consists of a network of interconnecting sieve tubes. In spite of its obvious agronomic importance, little is known about the first step of sugar entry (loading) into the sieve tubes. Here I propose a model which formulates several of the established, but difficult to reconcile, characteristics of the sieve tubes into a unifying model for sugar uptake. These characteristics include the alkaline sap¹ (pH 8–8.5) with a high potassium ion and ATP² concentration, the presence of electrical gradients³ and the considerable ATPase⁴ activity of the sieve tube plasmalemma. According to this model, sucrose leaves the site of synthesis in the mesophyll cells and enters the free space or apoplast which has a pH of 5 to 6. The sucrose is then accumulated into the alkaline sieve tubes by an active process involving membrane sulph-hydryl groups⁵. In this model the proton gradient of nearly 3 pH units across the sieve tube plasmalemma provides the driving force for sucrose uptake (Fig. 1). Based on non-electrolyte transport studies in bacteria⁶, algae⁷ and fungi⁸ it is proposed that protons are cotransported with sucrose into the sieve tubes.